Chemistry Reference
In-Depth Information
potentiometrically
24-25
. Besides utilising a photometric or spectrophoto-
metric reflectance method
26-27
, indigo can be determined by potentiometric
titration
23
.The difference in standard potential of indigo and sodium
dithionite is about 150 mV, which allows determination of both concentra-
tions in the same titration
23
. Most of these methods are not continuous,
which means that control of the indigo and sodium dithionite concentra-
tion cannot be done properly. Automation of these methods (e.g. with titra-
tion equipment) also resulted in only limited success owing to poisoning of
the detection system (e.g. potentiometric electrodes) and blocking of the
valves and the peristaltic pumps used for pumping the dyeing solution to
the analysis cell. Finally, photoacoustics were also investigated as a possi-
ble method for determining the concentration of dyestuff
28
.
In this chapter, a sensor system is described, based on multistep
chronoamperometry, that makes it possible to measure the indigo and
sodium dithionite concentration simultaneously, on line and in line and with
high precision, accuracy and long-term stability. This system allows the
quality and reproducibility of the dyeing process to be improved, because
the indigo and sodium dithionite concentration are measured and con-
trolled at a fixed value. In the following sections, the different steps that
were undertaken in order to develop this sensor are described in detail and
not only show the development strategy, but also give insight into the
working principle, advantages, disadvantages and properties of the sensor
system.
6.2
Basic electrochemical reactions of dithionite
and sulphite
A necessary pre-requisite to starting the development of a sensor system is
knowledge of the reactions that occur at the surface of the sensor electrode
and result in the response signal delivered by the sensor system. In this
section, the pathway of how the sensor reaction is found and studied is
described.
In a first set of experiments, the voltammetric behaviour of sodium
dithionite was investigated in alkaline solution (pH around 12.5), by varia-
tion of the rotation rate of the platinum-disc electrode for different con-
centrations of sodium dithionite. In Fig. 6.1, current-potential curves are
shown, obtained at different rotation rates of the electrode in a solution
with constant sodium dithionite concentration. Two anodic waves are
observed. In principle, sodium dithionite is the only electroactive species in
solution, therefore it is supposed that both well-separated waves can be
attributed to the oxidation of sodium dithionite with formation of a rela-
tively stable intermediate product in the first wave.
